Looks like nobody is using a Linux ARM based solution here.

I have now ordered MicroADSB (seems to be the cheapest solution at all) and will try how it works on my Raspberry Pi.
I will try the adsbox package (from http://diseqc.org.ua/projects/hard/adsb/index.html)

I would also like to feed to FR24 as soon as this works. Is there any documentation of the details of the upload format? I would release my scripts as OSS if they is any need for it. I will probably program this in perl.